Export implicit species

[RemDelaporteMathurin]

Proposed changes

Fixes #781 Requires #782

Types of changes

What types of changes does your code introduce to FESTIM?

• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Code refactoring
• [ ] Documentation Update (if none of the other choices apply)
• [ ] New tests

Checklist

• [x] Black formatted
• [x] Unit tests pass locally with my changes
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have added necessary documentation (if appropriate)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.39%. Comparing base (3fc02da) to head (3fc02da). Report is 19 commits behind head on fenicsx.

:exclamation: Current head 3fc02da differs from pull request most recent head b31a8a0

Please upload reports for the commit b31a8a0 to get more accurate results.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## fenicsx #783 +/- ## ======================================== Coverage 98.39% 98.39% ======================================== Files 28 28 Lines 1554 1554 ======================================== Hits 1529 1529 Misses 25 25 ```

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